Modular hvdc converter

ABSTRACT

A modular HVDC converter system including a high voltage direct current network, and at least two DC/AC converters being connected in series to the HVDC network. Each of the DC/AC converters is arranged to provide AC to a separate AC load.

THE FIELD OF THE INVENTION

The present invention relates to a High Voltage Direct Current (HVDC)converter system and in particular a modular HVDC converter system itscomponents and use thereof

BACKGROUND OF THE INVENTION

Feeding power to or from remote or awkwardly situated distributedinstallations from or to an existing power network creates slightlydifferent challenges compared to normal installations. This isespecially true for installations with several high power sources orloads. Examples of such remote or awkwardly situated distributedinstallations are offshore installations for oil and/or gas productionespecially installations sub sea, offshore wind mill installations, andmining districts or process plants at remote locations.

As is disclosed in EP 1385259, WO 0152379 and WO 0148892 it is favorableto use HVDC transmission to feed power to and from such installations.

The advantages of DC transmission have been commercially exploited since1954 when the first HVDC transmission was commissioned. Mercury-arcvalves were eventually replaced with high power thyristors and dctransmissions have reached several GW, over +/−600 kV, and distancesaround 1000 kilometres. In 1997, a new breed of HVDC converter stationsand HVDC transmissions were introduced. ABB has named its product familyHVDC Light®.

The use of forced-commutated power semiconductors in a voltage-sourceconverter allows much increased control of the active power flow,reactive power flow and harmonics when connected to even weak ac grids.This is well-known from low-voltage applications. The key factorallowing voltage-source converters to be connected to networks atvoltage levels hitherto unreachable, is the series-connection of powertransistors—in ABB's case IGBTs.

HVDC Light® can be connected to underground/submarine cables or overheadlines, on the dc as well as the ac side. A significant differencebetween classic HVDC and HVDC Light® is that in the latter, the dc-linkvoltage polarity is constant, irrespective of the direction of powerflow.

The present Light® technique, e.g. represented by EP 1385 2589 A, is agood solution for remote or awkwardly situated distributed installationsregarding controllability, low operating cost but also size andinvestment cost is within reasonable levels. In this solution, the powerto the installation is fed through HVDC cables and a HVDC Light®converter operates as AC power supply for a local AC network used todistribute the power at the installation, e.g to electric motors etc.Back up power is often provided via a local power supply using gasturbines or the like. However, such local power supplies requireconsiderably space and offshore manning.

SUMMARY OF THE INVENTION

The object of the invention is to provide a new HVDC converter systemwhich overcomes the drawbacks of the prior art. This is achieved by themodular HVDC converter system, its components and use thereof as definedin the independent claims.

Some advantages with the HVDC converter system according to theinvention are:

Each one of the modular converters is small sized and light compared toconventional converters, thereby the handling will be simpler,especially handling during maintenance and reparations.

The modular approach gives considerably better redundance compared withconventional converters, as the failure of one converter will not leadto full loss of electrical power, and loads connected to otherconverters will not be affected.

The modular converters can provide AC power of variable frequency, andare thus capable to function as drive units for electric motors.

These issues become even more important for offshore installations andin particular for installations subsea.

Other advantages are apparent from the following detailed description ofthe present invention.

Embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail below with reference to thedrawings, in which:

FIG. 1 shows a general circuit diagram of the modular HVDC convertersystem according to the present invention.

FIG. 2 shows an alternative circuit diagram for the HVDC convertersystem according to the present invention.

FIG. 3 shows a circuit diagram of one embodiment of a HVDC converter.

FIGS. 4 a to 4 c show three examples of rectifier stations or HVDC powersources arranged to feed DC current to a modular HVDC converter systemaccording to the present invention.

FIG. 5 shows a modular HVDC converter unit according to the presentinvention.

FIGS. 6 a and 6 b schematically show a layout of a plug in socket and amating plug in converter, respectively.

FIGS. 7 a to 7 c schematically show a plug in socket and a mating plugin converter.

FIG. 8 shows an alternative plug in socket and plug in converterarrangement.

FIGS. 9 a and 9 b schematically show one alternative embodiment of theconverter and a coupling scheme for interconnection of a plurality ofsuch converters.

FIG. 10 shows still an alternative circuit diagram for the HVDCconverter system according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically shows one embodiment of a modular HVDC convertersystem 1 according to the present invention. The modular HVDC convertersystem 1 comprises, a High Voltage Direct Current (HVDC) network 2, twoor more DC/AC converters 3 being connected in series to the HVDCnetwork, wherein each one of the DC/AC converters is arranged to provideAC to a separate AC load 4. The modular converter is built up of anumber of small sized series connected converter units 3, eachcontrolled individually. Failure of one converter will not stop theentire transmission as the others will continue to operate as normal.The converter units 3 are connected in series, and assuming an idealsituation wherein they represent identical loads in the HVDC circuit 2,the voltage across each converter unit will be approximately the HVDCvoltage divided with the number of active converters. HVDC is suppliedto the HVDC network e.g. by a rectifier station arranged to supply aconstant current (Id) at a variable voltage (V). The HVDC networkoperates at voltage in the range of 50 to 500 kV. In FIG. 1, the HVDCpower is supplied by a HVDC converter 5 connected to a three phase Highvoltage AC network 6, but the HVDC power could obviously be supplied byalternative means, such as a power plant etc. FIGS. 4 a to 4 c showthree examples of rectifier stations or HVDC power sources of which FIG.4 a shows a HVDC Light® type converter, 4 b shows a rectifier diodebridge type converter, and 4 c shows a thyristor bridge of classic HVDCtype. In the disclosed embodiment, the HVDC network is a bipolarnetwork, but it may also be a monopolar network, wherein the earth isused as return path, whereby cable costs are reduced.

According to one embodiment the converter, units 3 may be connected totheir respective load 4 such that the loads 4 can be shifted between twoor more converter units 3, 7, whereby the provision of one or morereserve converter units 7 will lead to significantly enhancedredundancy. One example of a modular converter system 1 with enhancedredundancy is shown in FIG. 2, wherein a reserve converter unit 7 can beconnected to any one of the loads 4 in case of failure or maintenance ofits associated converter unit. In FIG. 2 the reserve converter unit 7 isconnected to reserve power conductors 8, in turn connected to reservepower switches 9 that enable connection of each one of the AC load withthe reserve power conductors.

An alternative example of providing improved redundancy is disclosed inFIGS. 9 a and 9 b, wherein each converter unit 3 is provided with twoHVDC inputs and outputs respectively, and the units are connected toeach other in an overlapping manner as is shown in FIG. 9 b.

According to one embodiment, individual control of the modularconverters can be used to create variable frequency and voltage for amotor connected to the AC side and operate as a complete drive system orthey can be used to feed auxiliary power at constant voltage andfrequency. Hence, any additional drive systems can be omitted.

The individual converters could either be isolated from each other onthe ac side feeding different loads with possibly different frequencyand voltage levels or connected to a common bus as network supplies or acombination thereof through a normal low voltage switchboard.

The Modular DC/AC Converter

FIG. 3 shows one example of a modular DC/AC converter 3 according to thepresent invention. The DC/AC converter comprises a DC input 10 and a DCoutput 11 for connection in series with a HVDC network, AC outputs 12for connection to an AC load, a DC/DC converter 13 arranged to convert apart of the HVDC voltage in the HVDC network to a controlled voltageMedium Voltage Direct Current (MVDC), and a DC/AC converter 14 arrangedto convert the controlled voltage MVDC to a controlled voltage AC forfeeding the AC load. In the disclosed embodiment, the DC/DC converter 13is comprised of: a high voltage switch unit (HV switch) 15 that isconnected in series with the DC inputs, arranged to provide a controlledDC voltage over a capacitor 16 connected in parallel with the HV switch15 via a diode 17 on the input side. The DC/AC converter 14 is connectedin parallel with the capacitor and will thus have a controlled voltageMVDC on the input side. According to one embodiment, the HV switch is anIGBT switch 22 connected in parallel with a diode 23. According to oneembodiment, the DC/AC converter is a two level HVDC Light® ConverterBridge, and the load may be connected to the DC/AC converter by atransformer 19 to reduce the voltage fed to the load. The modular DC/ACconverter 3 further comprises a bypass switch 18 in order to bypass theconverter 3 in case of malfunction etc.

As mentioned above the DC/AC converter 3 may be arranged to providecontrolled voltage AC with variable frequency and voltage for a motorconnected to the AC side, whereby the converter can operate as acomplete drive system for a motor.

The dc voltage across the HV switch 15 in the DC/DC will vary with theAC load and become zero at no load and equal to the capacitor voltage atmaximum load. The switching voltage, i.e. MVDC for the converter will beconsiderably lower than the total dc voltage depending on how manymodular converters are connected in series. According to one embodimentthe MVDC voltage is in the range of 5 to 50 kV

According to one embodiment, in order to reduce size, switching is donedirectly towards the transformer 19 without using phase reactors and ACfilters. This is expected to be possible since the switching voltage isconsiderably lower than for a normal HVDC light converter. Thetransformer also has to be able to hold the dc voltage since theconverter will be located on high potential versus ground. If motors aredirectly fed by the converter, extra AC filters are not expected to beneeded however if auxiliary power is fed, a small AC filter might beneeded on the low voltage side of the transformer.

In order to fully take advantage of the modular converter system 1,according to one embodiment, the connection and disconnection of themodular converter(s) 3 is simplified. Moreover, the converter connectionon the dc side preferably is formed to allow for maintenance in andmanned intervention of one converter without affecting operation of theothers. This can be done in different ways where one embodimentcomprises a centralized high voltage dc switchboard and another ismodularized and integrated in the converter modules. Still anotherembodiment, which could be beneficial in a sub sea environment,comprises a separate plug-in connection box with the connectors close toeach converter module. The same principles can be used for both AC andDC connections.

Hence, according to one embodiment, there is provided a modular HVDCconverter system, comprising two or more modular HVDC converter units 3,FIG. 5, each comprising HVDC connection means 10, 11 for detachableconnection to a HVDC network 2 in series with at least one additionalmodular HVDC converter unit 3, AC connection means 12 for detachableconnection to a local AC network 4, and a current converter 20 forconverting AC to DC or DC to AC. In order to bypass the converter incase of malfunction of the converter itself or the load connectedthereto, the converter unit may comprise a HVDC bypass switch 18. Tofacilitate bypass of the converter unit, the bypass switch 18 can beremote controllable, which is especially suitable in situations wherethe converter unit is awkwardly situated.

According to one embodiment, the HVDC connection means 10, 11 and the ACconnection means 12 are provided in a plug-in arrangement, enablingplug-in attachment in a converter unit socket. One schematic example ofa plug-in arrangement is shown in FIGS. 6 a and 6 b. FIG. 6 a shows theplug-in configuration of a converter unit socket 30, the arrangementcomprising HVDC connectors 31, 32, AC load connectors 33, controlconnectors 34, and a switch actuator 35. FIG. 6 b shows theconfiguration of a plug-in arrangement of the connectors of a HVDCconverter unit 3 that mate the plug in socket arrangement of FIG. 6 a.In FIGS. 6 a and 6 b control connectors 36 and corresponding socketconnectors 34 have been included, the control connectors 34, 36 enablingcontact between the HVDC converter unit and a control network fortransmission of control signals to and from the converter unit. However,the transmission of such control signals could also take place withoutuse of a dedicated control network, such as by transmission ofsuperimposed signals in the HVDC network, or by wireless transmission.

FIGS. 7 a to 7 c show the connection of a HVDC converter unit 3 to aconverter unit socket 40. In this embodiment, the converter unit socketcomprises a schematic automatic switch arrangement 41, arranged tobypass the socket 40 when no converter unit 3 is arranged therein. Theswitch arrangement 41 is comprised of a switch actuator 35, a transversemember 42, two connection switches 43 and a bypass switch 44. Thetransverse member 42 is arranged to control the connection switches 43and is limited to movement in the horizontal plane. The transversemember 42 is biased in the direction that opens the connection switches43 and is provided with an inclined control surface 45 that interactswith a mating surface 46 on the switch actuator 35. The switch actuator35 has a protrusion 47 arranged to effect connection and disconnectionof the bypass switch 44. As can be seen from FIGS. 7 b and 7 c, theconnection switches 43 are initially connected by the transverse member42, and thereafter the bypass switch 44 is disconnected by the switchactuator 35.

By the provision of the plug-in arrangement, the connection anddisconnection of the modular HVDC converter unit in the modular HVDCconverter system, will be extremely simple and fast. Such an plug-inconverter unit is especially suitable for sub-sea conditions, wherebythe plug-in arrangement preferably is formed to allowconnection/disconnection of modular converter units from the surfacewithout need for personnel on site sub-sea. However, the plug-inarrangement is also beneficial in many other installations where e.g.quick replacement of defect converter units is required.

In order to facilitate installation of the modular HVDC converer systemaccording to the present invention, a plurality of plug-in converterunit sockets may be integrated as one socket with a plurality of plug-inpositions for converter units 3. FIG. 8 shows an embodiment such of aplug-in converter unit socket comprising three plug-in positions.

As already mentioned, the modular HVDC converter unit according to thepresent invention is very suitable for sub-sea installations, such asprecompression of natural gas reserves. Hence, for such installationsthe converter unit is provided with a housing, dimensioned to withstandsub sea conditions. Moreover, according to one embodiment, the HVDCconnection means and the AC connection means are of sub-sea type,enabling sub-sea connection and disconnection.

According to one embodiment the current converter is a DC/AC converterand the modular HVDC converter unit is arranged to drive an AC load inthe local AC network. As discussed above, the DC/AC converter can bearranged to provide AC with variable frequency, in order to providedirect control the frequency of rotation of an AC load in the form of anelectric motor. In one embodiment, the modular HVDC converter unit iscomprised as a part of an electric drive motor arrangement, e.g.arranged to drive a compressor unit at an offshore installation. Themodular HVDC converter unit according to the present invention is verysuitable to provide electric power to a cluster of compressor units atan offshore location, in that the system has a high level of redundancy.Redundancy in motor drives are normally not required since the motorsand compressors are often made in steps and loosing one step is notunusual and does not stop operation of the other motors. Howeverredundancy is often required for normal platform supply, which easilycan be achieved if two converters are used to feed auxiliary power. Thepower that needs redundancy is typically 10-20% of the total powersupply. According to one embodiment, the electric drive motorarrangement comprises a converter unit socket according to above,whereby the advantages related to simple connection and disconnection ofthe modular HVDC converter as discussed above are achieved. Theconverter unit socket may further be provided as an integrated part ofthe drive motor arrangement.

According to another embodiment, schematically shown in FIG. 10, thecurrent converter is an AC/DC converter 20 and the modular HVDCconverter unit is arranged to supply power generated in the local ACnetwork to the HVDC network. According to one embodiment, the AC powerin the local AC network is supplied by an AC generator arrangement 21.Like in the motor arrangement above, a modular HVDC converter unit canbe comprised as a part of generator arrangement 21. The generatorarrangement 21 may e.g. be arranged in a wind turbine unit, a wave powerunit, or the like and is especially suitable for clusters of such unitsarranged at remote locations, such as on offshore locations.

1-36. (canceled)
 37. An DC/AC converter, comprising: a DC input and a DCoutput for connection in series with at least one additional modularDC/AC converter in a HVDC network; AC outputs for connection to an ACload; a DC/DC converter arranged to convert a part of the HVDC voltagein the HVDC network to a controlled voltage medium voltage directcurrent; and a DC/AC converter arranged to convert the controlledvoltage medium voltage direct current to a controlled voltage AC forfeeding the AC load.
 38. An DC/AC converter according to claim 37,wherein the DC/DC converter comprises a high voltage switch that isconnected in series with the DC inputs, arranged to provide a controlledvoltage DC over a capacitor connected in parallel with the high voltageswitch via a diode on the input side, and the DC/AC converter isconnected in parallel with the capacitor.
 39. An DC/AC converteraccording to claim 38, wherein the HV switch is an IGBT switch.
 40. AnDC/AC converter according to claim 37, wherein the DC/AC converter isarranged to provide AC voltage with variable amplitude and frequency.41. An DC/AC converter according to claim 37, wherein the DC/ACconverter comprises a transformer directly connected to the AC side ofthe DC/AC converter.
 42. An DC/AC converter according to claim 37,wherein the HVDC network voltage is from 50 to 500 kV.
 43. An DC/ACconverter according to claim 37, wherein the medium voltage directcurrent voltage is from 5 to 50 kV.
 44. An DC/AC converter according toclaim 37, wherein the DC input and DC output comprise HVDC connectionsfor detachable connection to the HVDC network, wherein the AC outputscomprise AC connections for detachable connection to the AC load, theDC/AC converter further comprising: an HVDC bypass switch beingconfigured to interconnect the HVDC connections to bypass the DC/ACconverter.
 45. An DC/AC converter according to claim 44, wherein theHVDC connections and the AC connections comprise a plug-in arrangement,enabling plug-in attachment in a converter unit socket.
 46. An DC/ACconverter according to claim 44, further comprising: a housing,dimensioned to withstand sub sea conditions.
 47. An DC/AC converteraccording to claim 44, wherein the HVDC connections and the ACconnections comprise sub sea connections configured to enable sub seaconnection and disconnection.
 48. A converter unit socket, comprising:mating connection terminals configured to receive one or more DC/ACconverters comprising a DC input and a DC output for connection inseries with at least one additional modular DC/AC converter in a HVDCnetwork, AC outputs for connection to an AC load, a DC/DC converterarranged to convert a part of the HVDC voltage in the HVDC network to acontrolled voltage medium voltage direct current, a DC/AC converterarranged to convert the controlled voltage medium voltage direct currentto a controlled voltage AC for feeding the AC load, and an HVDC bypassswitch being configured to interconnect the HVDC connections to bypassthe DC/AC converter, wherein the DC input and DC output comprise HVDCconnections for detachable connection to the HVDC network, wherein theAC outputs comprise AC connections for detachable connection to the ACload, the DC/AC converter further comprising:
 49. A converter unitsocket according to claim 48, wherein the connection terminals aresub-sea connection terminals.
 50. A converter unit socket according toclaim 48, further comprising: a bypass switch and connection switchesfor the HVDC connection.
 51. A converter unit socket according to claim48, wherein the bypass switch is mechanically opened by the DC/ACconverter when placed in the socket.
 52. A modular HVDC convertersystem, comprising: a high voltage direct current network; and at leasttwo DC/AC converters connected in series to the HVDC network, whereineach one of the DC/AC converters is arranged to provide AC to a separateAC load, and wherein at least one of the DC/AC converters is a convertercomprising a DC input and a DC output for connection in series with atleast one additional modular DC/AC converter in a HVDC network, ACoutputs for connection to an AC load, a DC/DC converter arranged toconvert a part of the HVDC voltage in the HVDC network to a controlledvoltage medium voltage direct current, and a DC/AC converter arranged toconvert the controlled voltage medium voltage direct current to acontrolled voltage AC for feeding the AC load.
 53. The modular HVDCconverter system according to claim 52, wherein HVDC is supplied to theHVDC network by a rectifier station arranged to supply a constantcurrent at a variable voltage.